The present invention relates to a process for the chemical conversion and removal of nitrogen oxides from gaseous streams containing a stoichiometric amount of ammonia using a catalytic carbonaceous char.
Nitrogen oxides are the by-products of combustion process such as fossil fuel fired industrial boilers, electric utility boilers, as well as automobiles. In recent years, the regulation of nitrogen oxides has become more stringent due to the effects of nitrogen oxides on the environment. Nitrogen oxides are considered precursors to acid rain and contribute to the formation of ozone and haze. Nitrogen oxide emissions have been controlled by low NOx burners as well as selective catalytic reduction (SCR). The use of carbonaceous materials for removal of nitrogen oxides has gained attention due to the possibility of reduced costs when compared to either SCR or low NOx burners.
The use of activated carbon and other carbon containing materials has been recognized in the literature as a viable process for removal of nitrogen oxides from various gas streams. Rubel et al. describe the use of activated carbon between temperatures of 25xc2x0 C. and 125xc2x0 C. where nitric oxide (NO) was converted to nitrogen dioxide (NO2) in the presence of oxygen and activated carbon.
Ninomiya et al., U.S. Pat. No. 4,210,628 describes the use of an oxidized activated carbon for the removal of nitrogen oxides in the presence gaseous ammonia at temperatures ranging from 110xc2x0 C. to 350xc2x0 C. The nitrogen oxides are converted to nitrogen under these conditions.
Jxc3xcngten et al. describe the use of active coke and activated carbon which are preoxidized and incorporate heteroatoms of nitrogen through the exposure of the oxidized material to ammonia at high temperature. The use of such materials is shown to enhance the removal of nitrogen oxides in the presence of the modified activated carbon.
Illxc3xa1n et al. describe the use of activated carbon containing transition metals such a chromium, iron, cobalt, nickel, and copper. The addition of transition metals enhances the ability of the activated carbon to catalytically remove nitrogen oxides from gaseous streams.
All of the prior art for improving the removal of nitrogen oxides from gaseous media has certain disadvantages, which make the process unattractive from a commercial standpoint. Chief among these is an inability to determine in a rapid and convenient manner the suitability of a char for such applications prior to its use, in particular the intrinsic catalytic activity of the char for nitrogen oxide conversion. As a result of this shortcoming, it is not possible to know or even to estimate during the preparation of a char the utility of the final product short of actual testing in the application itself. None of the measures of typical char properties, e.g. iodine number and apparent density, has ever shown a clear correlation with utility in these applications, although some are known to affect overall reaction rates, primarily as a result of mass transport effects. This can be seen more clearly when several chars possessing nearly identical physical properties are contacted with a given nitrogen oxide-containing process stream, yet show significantly different rates of nitrogen oxide conversion and removal.
Accordingly, it is the object of the present invention to provide an improved process for the catalytic chemical conversion and removal of nitrogen oxides in gaseous media by contacting said media with a carbonaceous char in which the intrinsic catalytic activity of the char is measured and known prior to use. It is further the object of the present invention to estimate the intrinsic catalytic activity of the char using a test which is relatively cheap, quick, and simple in its execution and fairly indicative of the suitability of the char for the intended application.
In general, the present invention comprises an improved process for the catalytic chemical conversion and removal of nitrogen oxides from gaseous streams containing a stoichiometric amount of ammonia by contacting such process streams with a carbonaceous char. The present invention provides for the use of a carbonaceous char, which can rapidly decompose hydrogen peroxide in aqueous solution. More specifically, the carbonaceous char is preferably the low temperature char described in U.S. Pat. No. 6,342,129, which is incorporated herein by reference. Surprisingly, when tested under conditions wherein those char properties known to affect nitrogen oxide conversion and removal are held nearly equivalent, e.g. under conditions of nearly equivalent apparent density and iodine number, the rate at which the char can decompose hydrogen peroxide has been found to provide a fair indication of the utility of the char for nitrogen oxide conversion and removal.
The rate of hydrogen peroxide decomposition is measured by the test described in U.S. Pat. No. 5,470,748 and is reported, except where noted, as the t-xc2xe time, measured in minutes. In the present invention it is found that chars having the highest utility for nitrogen oxide conversion and removal are those having t-xc2xe times of 15 minutes or less, preferably 10 minutes or less. Other advantages of the invention will become apparent from a perusal of the following detailed description of presently preferred embodiments.